The unsettling story of a (literal) thought experimentby Philip Ball / March 16, 2018 / Leave a comment
Published in April 2018 issue of Prospect Magazine
Scientists are turning a sample from my arm in to a second brain. Photo: Prospect We’re used to thinking of ourselves as a finished product. The finish might not always be quite what we’d like, but it’s what we’re stuck with. From a single fertilised egg, we unfold in a progressive elaboration of cells and tissues until we come mewling and puking into the world. From there it’s a linear story that ends sans teeth, sans eyes, sans taste, sans everything. But Shakespeare’s age-old narrative of decay, decrepitude and ultimately oblivion no longer works. We have the means to rebuild and replace failing tissues. I speak from personal experience. Over the past several months I have seen a piece of my flesh that was cut from my arm develop into a structure called an “organoid,” a miniature organ. In my case it has become a structure that some call a mini-brain: the size of a frozen pea, it displays many of the distinct features of a real brain that grows in a foetus. I’ve seen evidence that the neurons in such tissue can fire, signalling one another. It would be too poetic to call these signals thoughts, but they are the stuff of thought. My flesh could have become something else, had the scientists so chosen. It could have become a kidney organoid, or one resembling a piece of heart or pancreas. It could have developed into light-sensitive tissue like that of the retina. And here’s the ultimate fact: it could have become an egg, or sperm, or something like an actual embryo, the beginnings of a being. It could have become any part of “me,” or every part. Here, then, is technology to stir tempting thoughts of cheating death, by replenishing the ailing body or even making a new, lab-grown self to “replace” the old. In the bicentenary year of Mary Shelley’s Frankenstein it would be easy to wax Gothic, if not apocalyptic, about all this: to imagine, say, people grown to order in vats like the Central Hatchery of Aldous Huxley’s Brave New World. But my mini-brains—there are several of them—were grown in a good cause. They are part of a project called Created Out of Mind, funded by the Wellcome Trust and aimed at improving our understanding of dementia and how we care for those living with it. The researchers who made the organoids are examining the genetic underpinnings of the neurodegenerative conditions that cause dementia. My mini-brains will be used in that research and so may, perhaps, help one day to defer the moment where other brains begin to close down. *** There are animals, like the salamander, that can regrow a whole lost limb comprised of tissues of many sort. We humans can grow skin to cover minor wounds, but otherwise the best we can do is the crude patchwork of scar tissue. And if an organ fails, that’s the end of it. We might survive with a donor graft or a mechanical replacement. But the culturing of flesh into different cell types and perhaps ultimately into entire miniature organoids are now bringing salamander-like regeneration within our grasp. These techniques not only have staggering potential in medicine, but they also overturn many years of received wisdom. If it seems unsettling, the reason is that we haven’t processed the truth that Frankenstein forced us to confront: that we are made from matter that somehow transcends itself to create a mind that peers out from its shell. We still don’t really know where, in that carnal being, to pin the flag of identity. The new sciences of “cellular reprogramming” are destabilising the issue as never before, as I have had brought home to me in a literally visceral way. Back last July, the tissue was sliced from close to my right shoulder by researchers at the Institute of Neurology at University College, London. With a little local anaesthetic, I didn’t really feel a thing. The crucial part of this biopsy was the layer of skin cells under the surface. These are called fibroblasts, the basic generators of connective tissue in the body. They make skin and are the key cells involved in wound healing. My fibroblasts were taken by neuroscientists Selina Wray and Christopher Lovejoy at UCL and allowed to proliferate in little Petri dishes of red solution that contain essential nutrients for cell growth. Two months later, I was able to look through the microscope at a fibroblast colony growing from the dark mass of my arm flesh, these elongated cell structures streaming out in aligned ranks as if they had some place to go. So how truly novel was what I was witnessing? The basic ability to grow cells in culture is these days an old art. It’s worth acknowledging that it was once regarded as a wonder, possessed of great mystique. When, in 1912, the French surgeon Alexis Carrel first claimed to have fostered “immortal” cells from a chicken heart, the newspapers ran with headlines about death no longer being inevitable. Those headlines provied to be over-excited in the extreme. But the growing of a “mini-brain,” from cells harvested from my skin is an entirely different proposition from the now-routine culturing of harvested cells. Wray and Lovejoy have to turn my skin fibroblasts into neurons: brain cells. They do this in two stages: first, by transforming them into a kind of cell that can develop into any tissue, and then by guiding them towards that particular fate. To grasp how the process works, you need to know that all living cells in a person contain exactly the same full set of “instructions”—encoded in DNA, which is arranged in 23 pairs of chromosomes and divided into regions called genes, each of which serves a particular function in our biochemistry. In principle, then, every cell has the same full code as every other. In practice, of course, different types of cells in a developed organism have different jobs to do. To accomplish this, different genes are switched “on” and “off.” This switching is what makes a cell one type (a brain, skin, muscle, liver cell and so on) rather than another. Much of this gene switching (or “regulation”) is done by protein molecules called transcription factors. These are themselves encoded in genes: that’s to say, the genome itself contains instructions for making the transcription factors that regulate it. Our cells make various transcription factors all the time to regulate the activity of genes. This enables different types of cells to behave differently. It’s also because of gene switching that a single fertilised egg cell can develop into an organism made of many distinct tissues. _____________________ Now try: Cathy Rentzenbrink on a fate worse than death Joanna Bourke on how we’re dying online Prospect’s podcast with Joanna Bourke, Philip Ball and Cathy Rentzenbrink _____________________ The earliest cells in a growing embryo, called embryonic stem cells, can become any tissue type: they are said to be “pluripotent,” and you could say that they still have all their genetic potential. But as the embryo develops into a foetus and then a baby, cells become committed to a fixed fate with a distinct function—heart, liver, brain—all organised and in the right place. We can intervene in this programming of cell behaviour. That’s the objective of gene therapy, for example, where the aim is to correct a “faulty” gene by adding to cells an extra, small piece of DNA that encodes a properly functioning form of the gene in question. But growing a “mini-brain” out of flesh cut from my arm requires something more ambitious than “correcting” a detail of a cell’s genetic instructions. It begins with a wholesale rebooting of the cell, apparently resetting all those on/off switches that have assigned the cell to a particular fate. It turns out that this can be done by just a handful of particular transcription factors. Wray and Lovejoy insert the genes that encode and produce those factors—little stretches of DNA—into the cells from my arm, using gentle electrical fields that open up temporary holes in the cell membranes through which the extra DNA can slip. This biochemical message, posted by Wray and Lovejoy into my fibroblasts, instructed these cells to turn back into stem cells like those of the early embryo, capable of growing into any tissue type in the body. These are called induced pluripotent stem cells. Scientists have been making them from human cells since 2007. Before that, most thought it was impossible. *** The person who changed that view was Japanese scientist Shinya Yamanaka, who, because he had a background in clinical medicine rather than cell biology, was perhaps better able to think the unthinkable—and consider whether cells that had already differentiated might be reprogrammed into stem cells. As long ago as the 1960s, experiments on frogs had given the first indications that the “fixing” of cells might be reversed. British biologist John Gurdon took frog eggs, hooked out their chromosomes, and inserted chromosomes taken from adult frog cells. These eggs, it turned out, could then be fertilised and grow into tadpoles and frogs. The chromosomes that, in the adult cells, had been regulated (through all those chemical on and off switches) for a specialised function were apparently being rejuvenated inside the eggs so that they could once again direct the growth of all sorts of new animal tissue. Such transfer of chromosomes from adult cells was used in 1996 for the cloning of Dolly the sheep. Mindful of such earlier successes, Yamanaka began analysing the transcription factors that were being produced in embryonic stem cells. Perhaps, instead of figuring out quite what had happened to the chromosomes of differentiated cells to imprint their specific patterns of gene activity and then trying to undo it all, it might be enough just to add a new dose of these factors to “persuade” the cells that they were stem cells? It seemed a speculative hypothesis, and yet it worked. Yamanaka found that when the genes encoding some of these factors—eventually just four proved to be enough—were added to differentiated human cells, they would revert to a stem-cell-like state. The discovery opened the doors for making tissues and perhaps entire organs outside the body. If grown from the recipient’s own cells—from, say, fibroblasts in a piece of my arm—there would be none of the problems that come from rejection of a donor transplant by the immune system. What’s more, drugs could be tested for toxicity on artificial human tissues grown in the lab—bypassing the recourse to animal testing, which is not only controversial but sometimes of limited use, because other animals aren’t always good models of the human response. The practical potential of the discovery was tremendous. But Yamanaka had also discovered a more profound truth. Our tissues and bodies are more malleable than we thought. Your flesh and bones can be converted, one type to another. Bone could be made from breast, brain from blood. Suddenly the whole integrity of the human body seems undermined. Wait, though: it gets even stranger. *** Shortly before Christmas, six months into the process, Wray and Lovejoy showed me the stem cells that had been made from my fibroblasts. Gone were the elongated streamers I’d seen earlier. Now the culture dishes contained dense clumps of smaller cells. Using molecular tags that latch on to particular proteins and glow different colours when light is shone on them, the researchers could show that the genes characteristic of stem cells were now switched on (see left, bottom). The first step was complete; the next step was to guide the cells to become neurons. Generally, stem cells need to be guided towards a particular “fate” by chemical triggers: adding more transcription factors characteristic of the target cells, say. But neurons are relatively easy to make, since they seem almost the default option: if stem cells in the lab begin to differentiate spontaneously, there’s a good chance they’ll turn into neurons. In fact, I’d even seen some signs of this in my own sample. Here and there, a lone cell had become detached from a cluster. One such loner, I noticed, had started to sprout the long, thin branches that nerve cells have, which normally end in synapses where these neurons pass electrical signals to one another. If these induced stem cells just grew into clumps of identical neurons, there would be little justification in calling the resulting tissues “mini-brains.” Our brains are not like that at all. They are complex structures containing several different types of neurons that produce electrical signals. Other cells in the brain aren’t neurons—for example glial cells, which help structure the brain and conduct maintenance tasks. And there are neural stem cells, a kind of partly differentiated stem cell geared to making just the various cell types of the brain, which give the brain its ability to reshape itself to changing circumstances—and, sometimes, to partially recover functions that have been damaged. On top of the question of the sheer variety of cells in the brain comes the question of how they are all arranged. The brain contains distinct structures and, remarkably, mini-brains echo some of them. This organisation of the tissue shows that neurons and other types of brain cell intrinsically “know” how to arrange themselves in “brain-like” ways. Sometimes this arrangement involves actual cell motion: the cells crawl over one another to find their rightful place, usually alongside others of its type. But this self-assembly needs guidance, and developing organs in the embryo use their surrounding tissues as a frame of reference. Brain cells, for example, need such signals to know where to sprout a brain stem, or to distinguish front brain from back. Mini-brains have some structure, but don’t quite get the shape right. They sprout neural tubes, for example—but whereas in a real embryonic brain just one of them would appear and advance down the spine to make the central nervous system, mini-brains make several at random, as if searching for a spine that isn’t there. For this reason, some researchers reasonably object to calling a neural organoid a “mini-brain.” But if the organoids are not brains in the proper sense, they are doing their best to become them. And researchers who make them will probably create more truly brain-like structures as they find ways of mimicking some of the “directional” guidance in the Petri dish. My mini-brains have no such benefits—they will be a mere rough sketch of a brain. All the same, they’re alive. And the neurons can communicate with one another by sending out electrical signals, as Wray plans to show by using special techniques to reveal bursts of calcium ions released at the junctions of synapses, just as one sees in real brain tissue. I’m not personally troubled by the notion that these are “thoughts.” I’m more disturbed by knowing that all that is currently going on in my (real) brain results from—as far as we know—nothing more than this sort of activity. Growing organoids like mini-brains outside the body is potentially just the first step in rebuilding the body. An ability to culture tissues in the lab sounds useful, even life-saving—think of a lab-grown pancreas made from the cells of a diabetic person, but gene-edited to produce insulin. But full-grown organs need a blood supply, which we don’t know how to make in a cell culture outside the body. And some tissues grown in cell culture, such as brain tissue or heart muscle, can’t simply be inserted in place: they have to be fully integrated into the existing networks of cells, which we don’t know how to do. But researchers are now exploring the possibility of growing new tissues directly inside the body. This could involve the same techniques used to reprogram cells to a stem-cell-like state and then guiding them to develop new characteristics. Such “in vivo reprogramming” has already been carried out in mice: liver cells have been turned into pancreatic cells, for example, or heart fibroblast cells into beating muscle cells. But Wray and Lovejoy’s two-step process—turn an ordinary cell into a stem cell, then turn it into a cell of another type—brings risks if done directly in the body. Versatile stem cells might be prone to becoming cancer cells. But, remarkably, scientists have found that with the right combination of transcription factors and molecular signals, they might be able to cut out the stem-cell stage, and switch one mature cell type directly to another: to make neurons straight from blood cells, say. Instead of turning the clock back and then running it forward again along a different route, you just make a sideways jump to another type of flesh. Animal tests have been encouraging, and human trials for repairing damage to heart muscle are being discussed. The possibilities for this sort of in vivo cell reprogramming are mind-boggling. Our bodies might one day be able to regenerate, like those salamanders regrowing their lost limbs. Brain damage caused by injury or a disease like Alzheimer’s might be repaired by commandeering non-neuron cells in the brain (like glial cells) and transforming them to working neurons. Because those cells are produced in situ, it’s at least possible that they will better integrate into the surrounding network. And this seems to happen at least for reprogrammed heart muscle, so that it beats in synch with the rest of the heart. *** Quite aside from the medical potential, these discoveries would compel a re-envisioning of the living organism. If liver can become muscle, blood can become brain, skin can be turned into bone, how then should we think about our mortal coil? Sure, wounds can heal, hair can grow again—but we have come to believe that you only get one body. When cells become fully malleable, it’s no longer clear that is true. What, then, is identity—the biological me? It’s clearly not what genome-reading companies like 23andMe insist is what “makes you you,” namely your unique genetic sequence. You became you only by virtue of the way that sequence was constrained and selectively activated in different cells: by a process of unfolding, interpreting, and modifying the genetic information. After all, there isn’t enough information in your genome to fully define “you,” with all your quadrillions of unique neuronal connections shaped by contingency and experience as you develop and grow. Do I really now have multiple “brains,” or at least “brain-like structures”? I’m still not sure how to think about that notion. I believe I could in principle have a “spare” heart or liver made this way, but the brain is too tied up with experience, memory, emotion and character, for me to conceive of any other organ but my own as a vessel for my identity. The idea of a “second brain,” even before allowing for the highly compromised nature of my mini-brains, doesn’t make much sense. I guess that’s a relief. After all, once these organoids have served their purpose for Wray’s research, they will be discarded, and I don’t imagine I’ll feel that any of my identity is going with them. Nonetheless, it still feels spooky to see what a rather random piece of me can become, growing in a lab in central London. I find it hard to avoid the conclusion that there’s a kind of meta-me that is all the tissues one could generate from that very first fertilised egg that quickened (in my case) in October 1962. I am just one expression of that meta-me. My boundaries of personhood feel just a bit fuzzier than they did. And what if the process of growing mini-brains gets ever better, until we can give them a blood supply and the coordinates to properly organise their parts—until we could make something very much like a full-grown brain? It’s purely a (forgive me) thought experiment right now: we simply don’t have the capability, let alone the motivation or ethical justification. But it’s not obviously impossible. What moral and ontological status would a brain-in-a-dish have? If the person whose cells were used to make it subsequently died, would they “live on” in the Petri dish? Would we at some point need to wonder: who is in there?